Hot-plug, also known as hot-swapping, refers to the ability to add and remove components from a computer system while a computer's main power is on and have the operating system recognize the change. Hot-plug allows components to be inserted and removed without rebooting the system. Protocols that support hot-plug include PCMCIA, USB, FireWire, Fibre Channel, and SATA. Hot-plug components include USB drives, server hard drives, PCI-X or PCI Express expansion cards, PCMCIA cards, and some power supplies.
Hot-plug support from a system perspective requires a number of hardware and software mechanisms to be developed. First, the system needs to be able to detect when a component is inserted or removed. In addition, all electrical and mechanical connections must be designed such that neither the component nor the user will be harmed by inserting or removing it. Other components in the system must also be designed such that a hot-plug event does not harm their operation.
Simple hot-plug implementations usually require a shut down procedure to be performed prior to removal. Often such devices are not robust when the component fails, and these types of hot-plug operations are reserved for moving peripheral devices from one system to another, or for synchronizing data between a device and a computer. More complex hot-plug implementations usually contain enough redundancy such that even if a shut down procedure is recommended, operation would continue if a device were removed without executing the shut-down procedure. Hot-plug operations of these types are used for regular system maintenance or broken component replacement.
PCIe Express Module (EM) is an industry standard hot-swappable I/O add-in card, used for server applications. The model requires a number of electrical switches to control power and interface signals, referred to as side-band signals, on the host side of the interface to prevent damage to the card or host system during hot-plug operations. In addition to eight side-band signals, both the host and the EM require Presence Detect signals, making ten interface signals in total between the host and the EM.
In a blade server environment, tens of hosts, and several tens of industry standard EMs can be used. In the case of PCIe internal connectivity between the hosts and EMs, several hundreds of side-band signals may need to be rooted in the chassis mid-plane, causing complex and expensive mid-plane or back-plane design for a blade server.